Why does ice slip? A mystery of physics two centuries old

Who has not ever accidentally slipped on ice or snow? Its slippery properties are completely familiar to us, but the explanation for the slippery quality of ice has sparked a scientific controversy almost two centuries old. Now, by modeling the problem using molecular simulation, we have been able to get closer to the answer.

The importance of deciphering the phenomenon

Humans have used the sliding properties of ice or snow since time immemorial. Sometimes as a form of leisure and other times as a means of transport. In ancient China, during the Ming dynasty in the 15th century, they created an ice road to transport hundreds of kilograms of marble slabs from the nearby mountains to the Forbidden City in Beijing.

Today’s ice transportation serves more mundane and modest purposes, but understanding the origins of this familiar property remains important, whether it’s to improve athletes’ performance at the Olympics or to ensure the safety of cars during the winter. winter.

The controversy began in Victorian England

For almost two centuries now, scientists have been arguing about why ice is so slippery, without agreeing, in a passionate historical controversy that began in Great Britain in the middle of the Victorian era.

At that time, the last glacier in the British Isles had melted more than 10,000 years ago, and great scientists of the time set out to understand the causes of its slide down the mountains. Among other interests, this was a good opportunity to launch the recently created tourist industry of the winter resorts.

Thus, a team of geologists, biologists, physicists, and botanists, including Huxley, Henslow, Tyndall, and Hooker, traveled to study on site the behavior of the ice on the Grindenwald glacier, in Switzerland, in the year 1856. In this expedition they coined the term regelation, the property of ice to melt and refreeze easily, which is at the origin of the first investigations on the ice friction.

The lubricating layer of water

From those studies the most accepted hypothesis arose, which supposes that on the surface of the ice there is a layer of melted water that acts as a lubricant. But how does this layer of water form below the melting temperature? Michael Faraday, famous for his studies on electromagnetism, proposed in the mid-18th century that ice melting occurs spontaneously on its surface even below the melting point. But this hypothesis seemed to contradict the newly formulated principles of thermodynamics and was not well received.

Around the same time, James Thomson, Lord Kelvin’s older brother, discovered one of the most unusual properties of ice: its ability to melt under pressure, just the opposite of most substances, which crystallize with increasing temperature. Pressure.

Using this concept, John Joly and Osborne Reynolds, fathers of hydrodynamics, suggested that the pressure exerted by a skate on ice would cause it to melt, allowing it to slide easily.

Nearly a century later, in 1950, Philip Bowden, one of the founders of the modern science of tribology, or the science of friction, proposed that the melting was not due to increased pressure, but rather to the heat generated as the skate slid. on ice.

Observation of ice atoms

The fact is that modern techniques for observing the surfaces of materials, such as advanced microscopy or x-ray diffraction, are usually carried out under controlled laboratory conditions, which are far from the usual conditions under which a skate slides. on ice.

To replace the complicated laboratory experiments, in our research, from the Complutense University of Madrid, in collaboration with our colleagues from the Autonomous University of Madrid and the Maria Curie-Skłodowska University of Lublin, in Poland, we have simulated the movement of atoms as a solid slides across the ice surface. This allows us to ‘see’ the individual movement of each of the molecules in the system like in a movie and count how many of them have melted.

Sliding of a solid on ice on a molecular scale. The water molecules (in red color) form an ice crystal that can be appreciated by its orderly stacking in layers. Spontaneously in the vicinity of the surface with the solid (black in color), the ice has melted, which can be seen by its disordered appearance. We color the molecules in the center of the solid and the molten layer blue to see how far they have moved after five billionths of a second. As can be seen, when the solid slides on the ice, it drags the water molecules along, which causes friction.
Author provided

Our results, published in Proceedings of the National Academy of Sciences, show that Michael Faraday was largely right. As soon as the solid is brought into contact with a perfect ice crystal, the molecules closest to its surface are observed to immediately melt, indicating that the ice spontaneously forms a layer of lubrication. But Thomson and Reynolds were also right. Indeed, when we compress the solid against the ice, we observe that the layer of liquid water continuously increases its thickness the greater the pressure.

And there is still more: as soon as we put the solid to slide on the ice, we find that if the initial friction is great, the heat generated melts the surface a little more, increasing the thickness of the liquid layer and improving its lubrication, just as as Bowden proposed.

The result is a beautiful story about scientific progress. Sometimes when describing the advances in science we do as in sports reports and we focus only on the author of the shot on goal, but in truth the advances are always the result of a continuous effort by an entire scientific community whose contributions are frequently overshadowed by the reputation of the best-known authors.

They were all partly right

In this way, we have found that the main keys to the slippery nature of ice are the phenomenon of superficial melting proposed by Faraday; melting due to pressure, reminiscent of Thomson’s hypothesis, and melting due to friction proposed by Bowden. But, contrary to the Thomson and Bowden hypotheses, these phenomena are all melting processes that affect only the ice surface and therefore can occur even below its melting temperature.

This combination of factors endows the ice surface with a self-lubricating and self-repairing layer of water whose lubricating power is fed back as sliding speed increases.

In recent times, some experts have questioned the role that a layer of water can play as a lubricant for ice. In fact, water is a bad lubricant. Being very fluid, when we put it between two joints, the high pressure expels it, and the joints are in direct contact, generating a lot of friction. For this reason, the most commonly used lubricants are usually viscous and slightly fluid liquids, such as oil. In the case of ice, things are very different. At the same time that the pressure expels the water, the ice on the surface melts and repairs the loss, as an example of the well-known Le Chatelier principle.

The three main hypotheses that have been in dispute for so long are indeed mutually compatible and work simultaneously to give ice that slippery quality that makes it exceptional.

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